PDO mysql错误

I am currently working on a project for a client but because I am new to pdo I have no clue how to hand the error it keeps spitting out. The code I am working with is not mine either, so that adds a bit of confusion to the mix. It keeps telling me:

Query failed: You have an error in your SQL syntax;
check the manual that corresponds to your MySQL server 
version for the right syntax to use near '' at line 1

I have narrowed down the error to these lines:

$regid = $dbh->lastInsertId('');

$dupsid = true;
while ($dupsid){
    srand((double)microtime()*1000000);
    $maxrand = 100000000;
    $rand_sid = rand();
    $check_sid = "select reguniqid from v_events_registrants where reguniqid = :RAND_SID";
    $stmt = $dbh->prepare($check_sid);
    $stmt->bindValue(':RAND_SID', $rand_sid);
    $stmt->execute();

    $num_result = $stmt->rowCount();
    if ($num_result == 0) $dupsid = false;
}

$uniqid_upd = "update v_events_registrants set reguniqid = :RAND_SID where registrant_id = :REGID";
$stmt = $dbh->prepare($uniqid_upd);
$stmt->bindValue(':RAND_SID', $rand_sid);
$stmt->bindValue(':REGID', $regid);
$stmt->execute();

in this case here $reg is the primary key of the table in which the last few items were added. Initially I thought that was the issue but when I cleared it of ', and " I get an invalid id error, which I am guessing is from the next execution of the pdo. Please help as this error is really starting to hold me back from completing this project for my client.

Your first line defines $reg, then you try to use the undefined $regid

Nearly positive $regid is not defined, at least not within the scope of the code you included.

Trace that variable back or define at as something and you should be fixed.

As your $rand_sid is of type integer, both times you use bindValue for $rand_sid you should add the datatype PDO::PARAM_INT (it takes PDO::PARAM_STR as default), like this:

$stmt->bindValue(':RAND_SID', $rand_sid, PDO::PARAM_INT);

Most likely your error lies somewhere else.

So, first of all get rid ov any try..catch blocks in your code
Then turn error reporting on
Then run your code again and find the real place where error occurs from the stack trace.
Then you get to erroneous query, write it this way

select
reguniqid
from
v_events_registrants
where
reguniqid
 =
 :RAND_SID

and watch the line number - it will help you locate the problem spot.